The invention relates to electronic circuits and particularly to an electronic circuit adapted for use as a voltage source, and more particularly to voltage sources adapted to provide a regulated voltage.
The provision of a well controlled voltage at a low impedance and independent of supply voltage changes is important in many applications, especially high speed mixed signal Integrated Circuit (IC) designs. Although regulation usually implies a circuit that is immune to changes in the load impedance and current, there are applications where the load is well defined and behaved, one such example being biasing.
Standard regulators usually suffer from the problems of either poor power supply rejection, poor stability characteristics or high drop out, which means that the input and output voltages of the regulator cannot be close. Problems associated with prior art implementations arise in situations where it is required to implement the voltage source in circuits having low headroom as many of the prior art solutions utilize replicator type circuit incorporating current sources to provide the constant regulated voltage.
FIG. 1 shows a simple voltage source circuit 100 according to the prior art adapted to regulate the output voltage on detection of variations to an input voltage. A voltage input 105 which is usually the supply to the circuit is connected to the source 110 of a transistor 115 whose gate potential 120 is controlled by the output of an amplifier 125. The drain 130 of the transistor 115 is connected to an output voltage node 130, which maintains the voltage utilizing an resistor impedance 135. A first input 140 of the amplifier 125 is used to sense the output voltage and a second input 145 is a reference voltage input. The sensed output voltage is fed back to the amplifier and compared with the reference voltage, thereby controlling the gate voltage and maintaining a constant voltage output at a voltage node 130.
FIG. 2 illustrates an alternative prior art voltage source configuration 200 which uses a top current mirror (MN1, MN2) to ensure that the currents in Q1 and Q2 are equal (or directly proportional). If this is true, it can be readily shown that:
xe2x80x83I1=(Vt/R2)*1n(A)xe2x80x83xe2x80x83(equation 1)
where A is the ratio of the sizes of Q1 and Q2, Vt is the thermal voltage of a bipolar transistor and R2 is the value of the resistor R2. If equation 1 is satisfied it will be appreciated that the output voltage is independent of the input voltage Vi, being only a function of I1 and the parameters of R1 and Q1. This circuit suffers, however, in that when a load current is place on the output voltage positive feedback is employed, which magnifies the errors due to load current. In this case, the load current subtracts current from the impedance, and the current which is fed back (via the top mirror) decreases, thus supplying LESS current to the output.
Arising from the above mentioned shortcomings of the prior art techniques there is a need for a voltage source that is simple and easy to implement, can be used to achieve an output that is both independent of the input and has a low output impedance, utilizing real impedances independent of the gain of the circuit, is operable in circumstances having a load current applied to the voltage output and minimizes headroom requirements.
These needs and others are satisfied by the present invention. In accordance with one aspect of the present invention a regulated voltage source circuit is provided, the voltage source adapted to provide a regulated output voltage independent of variations in an input voltage and having an input node adapted to receive an input reference current and an output node adapted to provide a definable voltage output, the circuit comprising:
a control element adapted to provide an output signal to the output node,
an impedance being driven by the output signal of the control element,
a sensing element having a current mirror adapted to sense the current flowing through the impedance, and to provide a feedback signal, and wherein the control element is responsive to the difference between the feedback signal and the input reference current.
The feedback signal is desirably a negative feedback signal, such that when a load current is applied to the output voltage node, the sensed current flowing through the impedance drops and the control element increases the output signal to the output node. By employing a negative feedback response within the voltage regulator the error in the output voltage response due to applied load at that node is reduced.
The circuit may additionally include a current inversion element provided between the sensing element and the control element, the current inversion element adapted to invert the signal from the sensing element prior to driving the control element in order to achieve the negative feedback.
The current inversion element may desirably include a current mirror having a matching pair of transistors.
The sensing element may comprise one or more pairs of matching transistors forming the current mirror, the input of the current mirror being connected to the impedance.
The current mirror is desirably adapted to sense the current through the impedance and also contributes to that impedance.
The control element may include a current mirror, comprising at least one matching transistor pair.
The transistors are preferably selected from MOSFET or bipolar transistors.
The impedance may include a diode, a diode/resistor combination or equivalents.
The sensing element and impedance may be integrally formed.
The invention additionally comprises a voltage source circuit having an input reference current and providing an output voltage, the circuit comprising at least one impedance component in electronic communication with a control current source having an input and an output, the output driving the impedance components and the output of the voltage source. Sensing elements adapted to sense the current flow through the impedance components are additionally provided together with comparison elements adapted to compare the sensed current with respect to the reference current source. The comparison elements are adapted on detection of any variance between the sensed current and the reference current to vary the input to the input of the control current source thereby varying the current flow through the impedance and maintaining a regulated voltage output.
In one form of the invention the sensing elements comprise at least one current mirror, and the current mirror may form a portion of the impedance components.
Desirably the impedance components are temperature dependant such that the impedance of the components varies with temperature. Typically the components are selected from one or more of the following electronic components: diodes, bipolar transistors, MOS transistors, and/or resistors.
In a preferred embodiment the control current source is adapted to provide a variable signal. Any variations between the sensed current and the reference current can then be used to vary the output of the control current source so as to compensate for this variance.
The sensing elements are desirably further adapted to sense any voltage drop at the output of the voltage source due to a load there, which equivalently to the sensing of any variance between the reference current and the sensed current can be used to vary the current applied to the impedance so as to compensate for this voltage drop.
The comparison means desirably includes a current mirror whose output is compared to the input reference current.
The reference current may be Proportional to Absolute Temperature (PTAT), and it will be appreciated by those skilled in the art that by including components within the impedance element which are also temperature dependant that the circuit of the present invention can be made to have any desired temperature dependence, or be independent of temperature.
In a first embodiment the output of the voltage source is ground referred, i.e. current source is between the impedance and the input to the voltage source; alternatively the output of the voltage source is supplied referred: i.e. current source is between the impedance and ground; or alternatively the output of the voltage source is floating.